Introduction to Acoustics

Time T (s)
2.5
2
1.5
1
5
10 15
Acoustics in Halls for Speech and Music 9.6 Geometric Design Considerations 329
20
V/ST (m)
Fig. 9.28 Reverberation time T in occupied concert halls
versus the ratio between the volume V and area occupied
by the audience and orchestra ST (including aisle space up
to 1 m width). Derived from empirical data (after [9.9])
These guidelines on volume should be regarded as
rough rules of thumb only, as room shape, floor slope,
the presence of balconies and the distribution of absorption
on other surfaces can cause significant variations
in the objective values obtained for T and other parameters.
In smaller halls (say below 1500 seats) for
symphonic concerts, it is better to start with a volume
slightly larger than required rather than the opposite, as
one can always add a little absorption – but not more volume
– to a hall near completion. On the other hand, in
larger halls, where maintaining a suitably high G value
is also of concern, a better strategy is to limit the volume
and minimize the absorption instead. One way of minimizing
absorption is to place some of the seats under
balcony overhangs, where they are less exposed to the
reverberant sound field. However, the other side of the
coin is the poor conditions in these seats, as explained
in Sect. 9.6.3.
9.6.5 Main Dimensions and Risks of Echoes
In order for early reflections to contribute to the intelligibility
of speech or to the clarity of music, they must not
be delayed more than about 30 ms and 80 ms, respectively,
relative to the direct sound. If the first reflection
arrives later than 50 ms, it is likely to be perceived as
a disturbing echo when impulsive sounds are emitted
(Sect. 9.2).
In large rooms it is a challenge to shape the surfaces
so that reflections from the main reflecting areas arrive
within 50 ms at all seats. It is possible to identify which
surfaces are able to generate echoes at the receiver point
A
Stage
k
Source K L M N
Fig. 9.29 Echo ellipses drawn over the plan of an auditorium
(after [9.4])
in question by means of simple geometrical studies of
plan and sectional drawings as seen in Fig. 9.29. Ellipses
are drawn so that the source and relevant receiver positions
are placed at the focal points and so that the sum
of distances from the focal points to any point on the ellipse
equals the distance between the focal points plus
17 m (times the scale of the drawing). Then, if a surface
outside the ellipse marked “m” in the figure directs
sound towards seats near point M, this surface must be
made absorbing, diffusing or be reoriented so that the reflection
is directed towards areas farther away from the
source than M.
In particular, it is important to check the risk of
echoes being generated by the rear wall behind the
audience and from a high ceiling.
9.6.6 Room Shape Details
Causing Risks of Focusing and Flutter
Concave surfaces can cause problems as they may focus
the sound in certain areas while leaving others with too
little sound. Thus, vaulted ceilings as seen in Fig. 9.30
are only acceptable if the radius of curvature is less than
half the height of the room (or rather half the distance
from peoples’ heads to the ceiling) so that the focus
center is placed high above the listeners. Situations with
l
m
r < ½h r = ½h
h
r = h r = 2h r > 2h
Fig. 9.30 Focusing by concave ceilings (after [9.31])
h
n
Part C 9.6